Problem: At lunch, $60\%$ of the students selected soda while $20\%$ selected milk. If 72 students selected soda, how many students selected milk?
The number of students who selected milk will be $\frac{20\%}{60\%}=\frac{1}{3}$ of the number of students who selected soda.  Therefore, $\frac{1}{3}\cdot 72=\boxed{24}$ students selected milk.